Electronic devices face continued pressure to design and produce their configurations in a further state of miniaturization, ergonomically pleasing shapes, and a reduced weight. To achieve these goals, substrates must be thinned to 100 um (microns) and less, making them extremely fragile and difficult to handle with existing equipment. To prevent breakage, cracking, or otherwise chipping and stressing these fragile substrates, it becomes necessary to always keep them temporarily supported by an external platform, being a rigid carrier or a membrane. During microelectronic manufacturing, the thinned substrates are temporarily supported by rigid carriers, as these provide the most secure and reliable media to conduct high-resolution processes. These carrier substrates may be composed of sapphire, quartz, certain glasses, or silicon and exist in thicknesses from 0.5-1.5 mm (millimeters=500-1,500 um). The device substrate is commonly affixed to the carrier by an adhesive that offers sufficient adhesive force and quality to withstand the manufacturing process, while also allowing the thinned substrate to be removed at the completion of work without damaging to its integrity.
Common tape adhesives exist which offer temporary support to the device substrate either alone or used as an interface to the carrier. These materials are commonly used for dicing operations, including high-volume photodegradative delamination practices (i.e. pick-and-place). However, tape adhesives are reserved only for the end of the process where dicing occurs. Most tape adhesives are not used in upstream microelectronic processes as their properties do not meet the needs for fabrication, including rigidity and uniformity, thermal and chemical resistance, and outgassing (weight loss). These shortcomings in adhesive tapes result in loss of adhesion, gas bubbles lodged in-between the device substrate and carrier, or produce unwanted gaseous by-products of degradation, which will adversely interact, with the processes of vacuum deposition or etching to produce inferior results.
In the example where thinned substrates include semiconductor wafers, the device wafer is commonly removed from a carrier support, cleaned, and mounted to a film frame containing tape adhesive, allowing the dicing process to proceed. Carrier removal is conducted with robotic assisted complex tooling. Tooling is designed according to the type of adhesive chosen. At the time of this invention, there are no less than six (6) adhesive materials on the market. The majority of these adhesives require a single wafer tooling configuration whereby the tool handles one wafer at a time.
Single wafer processes that use thermoplastic adhesives may utilize thermomechanical demounting as taught in U.S. Pat. No. 6,792,991 B2, Thallner, and 2007/0155129 (2007), Thallner. Device wafer separation is achieved by heating the mounted stack to a temperature above the melting point of the thermoplastic adhesive while simultaneously applying a shear force in a manner designed to separate the mounted surfaces. Cleaning with a selected organic solvent typically follows to ensure residual adhesive is cleaned from the substrate.
Another single-wafer tooling practice for removing carrier supports is described in U.S. Patent Application Nos. 2009/0017248 A1 (2009), Larson et al., 2009/0017323 A1 (2009), Webb et al., and in the International Application WO 2008/008931 A1 (2008), Webb et al. The adhesive described is a bilayer system composed of a photothermal conversion layer and a curable acrylate. The applications cite the use of a laser irradiation device which allows rapid demount of the external support carrier and is followed by a mechanical peeling practice of the curable acrylate from the thinned substrate.
Additional laser ablative carrier demounting practices are described in U.S. Pat. No. 6,036,809, Kelly, et. al, U.S. Pat. Nos. 7,867,876B2, and 7,932,614B2, Codding, et. al. Laser ablative tooling is non-trivial, in that it requires exacting focus of an optical device of a specific wavelength and to do this onto an interface between the work unit and the carrier substrate. The laser's focus does this while it or the substrate is being shifted in continual motion moving rapidly across the substrate. It is well known to those familiar with the art of coatings and planarization efforts that irregularities will exist in materials applied over the surface of the work unit. The adhesives used for these practices vary between rubber, silicone, polyimide, acrylic, and the like. The laser transmits through an optically clear carrier substrate and focuses onto the interfacial region where the adhesive meets the carrier, causing a significant and immediate rise in temperature to burning of that material to destroy its adhesion to the carrier. There is a micro area of impact that absorbs this temperature rise and fall during contact. The laser continues to move to the next location in an apparent smooth fashion until the entire surface of the substrate has been exposed and thereupon the release of the work unit is expected. The impacts of this process is realized later when irregularities are observed as micro-cracks, fissures, and residue that is burnt onto surfaces which cannot be removed. Laser ablative processes, although a common practice for debonding delicate substrates, remains a subject of much discussion when considered for high volume manufacturing.
These and other carrier debonding (removal) practices are discussed in U.S. Patent Application No. 2009/0218560A1, Flaim, et. al, where the author consolidates the practice of wafer and carrier separation into four approaches. These include 1) chemical, 2) photodecomposition (laser ablation), 3) thermomechanical, and 4) thermal decomposition. Although the author mentions drawbacks in each mechanism, they refrain from classifying them as single-wafer or batch processing according to their respective tooling configuration. Of these four processes, only chemical penetration is considered as a batch mechanism. In such processes, wafers may be populated into a cassette or holder and immersed into a chemical liquid for a designated time to allow penetration into the adhesive, emulsification, and removal to allow carrier debonding. As mentioned in U.S. Patent Application No. 2009/0218560A1, Flaim, et. al, chemical debonding may require hours to complete. At the time of this document writing, common throughput for single wafer processes may vary between 8-12 wafers per hour (wph). In the case of a conventional chemical debond, cassettes of between 12-25 wafers are used and may last up to four (4) hours. For a bath size of >100 liters as common for most fabrication facilities in Asia, this volume can accommodate up to 4 cassettes at a time, providing throughputs between 12-25 wph, exceeding that for single wafer processes (i.e. 12-25 wafers per cassette×4 cassettes=48-100 wafers/4 hrs=12-25 wph). Without being bound to variations of the art of batch processing, this option is needed in fabrication to offer lower cost options for debonding carrier substrates. Therefore, it is a desire to consider batch wafer processing as a viable and cost effective practice for thin substrate debonding from carriers.
Batch debonding processes are described in U.S. Pat. No. 6,076,585, Klingbeil, et. al, and U.S. Pat. No. 6,491,083 B2, De, et. al, where a fixture holding thinned gallium arsenide (GaAs) wafers are removed from sapphire carriers using an immersion chemical practice. In both of these inventions, the fixture is designed to operate with the wafers held horizontally. The fixture has steps machined within it and requires a perforated carrier substrate that is slightly larger in diameter than the device wafer, such that during the debonding operation, the separation of the two substrates occurs by one item landing upon the fixture step while the wafer separates and falls to a lower level of support. Carrier substrates that are machined to be larger in diameter than the work unit and which have perforations can be costly. For example, enlarged perforated sapphire substrates are a common choice for GaAs work unit wafers, however, these can cost $1,000 or more for each piece. In the case of silicon substrates of diameters at 12″ or 18″, carrier wafers are chosen to be dummy type (i.e. same size, shape, and composition of the work unit without the electronic purity). Oversized perforated carriers are cost prohibitive for silicon processes as their cost can fall between factors of 10-100× that of conventional dummy sized wafers. It is a desire to avoid fixtures that require oversized perforated carriers and instead use fixtures that accept dummy wafers as carriers for thin wafer handling as a means to minimize process costs.
A batch demounting process is also described in U.S. Pat. Nos. 6,601,592 B1 and 6,752,160, Zhengming Chen, where two fixture cassettes work in conjunction with each other in a manner that allows separation of the device wafer from carrier substrates. The inventions describe the batch process separation between device wafer and carrier as conducted such that the top fixture cassette is populated with the mounted wafers whereby during liquid immersion, the chemistry penetrates the adhesive contact to release the two substrates. The top fixture cassette is constructed in a manner to allow only the device wafer pass downwards to the lower fixture cassette during gravity assisted separation, retaining the carrier substrate. The inventions require the sized of the carrier substrate and device wafer to be different, either the device wafer to exhibit a flat edge (i.e. wafer flat) or the carrier substrate to be oversized as compared to the device wafer. In either case, when the process commences and the fixture cassettes are arranged vertically, the oversize carrier is held back within the above fixture cassette while the device wafer travels from the top to the bottom cassette. Device wafers with a flat location were at one-time popular for reasons of reference location when handling and transferring from one process to another. The wafer flat is less desirable as it eliminates valuable device manufacturing realty on the wafer and reduces the number of devices built upon a substrate. Conversely, oversized carrier wafers are cost prohibitive as described earlier in this document. Further and most important, these inventions describe fixture design that requires the device wafer to be separated and released from the carrier substrate and move freely from one fixture cassette to another during liquid chemical immersion processing. It is commonly understood in the practice of thin wafer handling, that at anytime during this work, the device wafer should always be supported and never left to move freely. Consistent device wafer support would minimize irregular bending, vibration, and edge contact that would generate cracks, chipping, and other flaws within a thin wafer. It is a desire to avoid fixtures that require device wafer flat designs or oversized carriers and to avoid fixtures that promote a batch processing practice which allows the device wafer to move freely and subject itself to cracks, flaws, or other signs of breakage.
A unique carrier formation and process for separation from the device wafer is described in the International Publication No. WO 210/107851 A2 (International Application No. PCT/US210/027560), Moore, et al, where a carrier substrate is manufactured (formed) directly onto the device wafer in a manner sufficient to support grinding and backside processing and when complete, the materials used to form the carrier are designed to break down in a liquid chemistry cleaning process. Carrier supports which are removed by chemical breakdown during a special cleans process require a special fixture to allow the device wafer to remain in tact and without damage. These batch clean designs and would require a fixture that would support multiple device wafers during carrier removal. It is a desire to use fixture designs that promote a batch processing practice which allow the device wafer to be held secure while the carrier is allowed to be chemically broken down by a chemical fluid or otherwise be removed.
For these reasons and others not mentioned, it is a desire to perform batch process separation (debonding) of carriers from device wafers in a manner that accepts low-cost dummy wafers as carriers and maintains support of the device wafer throughout the process.